We intend to determine whether juxtamedullary nephrons possess the ability to exert feedback control upon their rate of glomerular filtration. In a series of open feedback loop experiments, we will measure tubuloglomerular feedback in juxtamedullary and superficial neuphrons in hydropenic and diuretic hamsters. The effects of Henle's loop perfusion rate and perfusate composition will be assessed. In a series of closed feedback loop experiments, we will measure and determine if tubuloglomerular feedback mediates juxtamedallary nephron filtration rate autorequlation. We will also determine if tubuloglomerular feedback leads to a suppression of juxtamedullary nephron filtration rate during diuresis. An integral part of our research will be a mathematical model of loop of Henle solute and water reabsorption. The model will be used to assist in experimental design and data interpretation.